1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same. In particular, the present invention relates to a semiconductor device using, as an active region, a crystalline silicon semiconductor film obtained by crystallizing an amorphous silicon semiconductor film formed on a substrate having an insulating surface, and to a method for fabricating the same. More specifically, the present invention is applicable to a semiconductor device using thin film transistors (TFTs) provided on an insulating substrate such as a glass substrate, an active matrix type liquid crystal display device, an image sensor, a three-dimensional IC and the like, and to the fabrication thereof.
2. Description of the Related Art
Recently, it has been attempted to form semiconductor devices having good performance on insulating substrates, made of materials such as glass, or on insulating films which are formed on a surface of a substrate, for realizing large liquid crystal display devices having a high resolution, close-contact type image sensors having a high resolution with high speed, or three-dimensional ICs and the like. Generally, the semiconductor devices used in these apparatuses are formed from silicon semiconductor thin films.
Such silicon semiconductor thin films are roughly classified into two types: amorphous silicon semiconductor films and crystalline silicon semiconductor films.
Of these two types, the amorphous silicon semiconductor film is preferred and enjoys general use because it has a low processing temperature and is easily manufactured using a vapor deposition method, thus lending itself to mass production. Compared to the crystalline silicon semiconductor film, however, the amorphous silicon semiconductor film is inferior in properties such as electrical conductivity. It is therefore strongly desired to establish an efficient fabrication method for semiconductor devices formed from crystalline silicon semiconductor films so as to achieve faster response characteristics of the semiconductor devices.
The crystalline silicon semiconductor films currently known include polycrystalline silicon, microcrystalline silicon, amorphous silicon containing crystalline components, semi-amorphous silicon having an intermediate state between crystalline and amorphous forms, etc. The following three methods are known for the production of these thin film crystalline silicon semiconductors.
(1) A first method in which a crystalline silicon semiconductor film is directly formed in a film deposition step. PA1 (2) A second method in which an amorphous semiconductor film is first formed, followed by crystallization of the amorphous silicon semiconductor film using a laser's optical energy and the like. PA1 (3) A third method in which an amorphous semiconductor film is first formed, followed by application of heat energy to crystallize the amorphous silicon semiconductor film.
However, these methods have the following disadvantages.
According to the first method, crystallization proceeds during the deposition step. Consequently, a thick silicon semiconductor film must be formed to obtain a crystalline silicon film with a large grain size. It is technically difficult, however, to form a film having good semiconductor properties uniformly over the entire surface of the substrate. Furthermore, the film needs to be deposited at high temperatures of 600.degree. C. or more. This introduces disadvantages in cost in that inexpensive glass substrates cannot be used since they have low softening temperatures.
The second method utilizes crystallization in melting and solidification processes, and allows the formation of a high-quality crystalline silicon semiconductor film with a small grain size and yet having properly treated grain boundaries. However, with the lasers commonly used today, such as excimer lasers for example, processing throughput is low because the effective laser beam radiation area is small. A further disadvantage is that the stability of the lasers is not sufficient to uniformly process the entire surface of a large substrate.
The third method has an advantage over the first and second methods in that a thin-film crystalline silicon film can be formed uniformly over a large substrate. The method requires, however, heat treatment at high temperatures of 600.degree. C. or more for several tens of hours to accomplish crystallization. Therefore, to allow the use of inexpensive glass substrates and to increase the processing throughput, contradicting requirements need to be satisfied simultaneously, i.e, to lower the processing temperature and to accomplish crystallization in a short period of time.
Methods for solving the aforementioned problems of a heat treatment utilizing method (3) above are disclosed in Japanese Laid-Open Patent Publication Nos. 6-244103 and 6-244104. These methods make it possible to reduce both the processing temperature and the time required for crystallization by utilizing catalyst elements for accelerating the crystallization of an amorphous silicon semiconductor film.
According to the proposed methods above, at least one of nickel, iron, cobalt and platinum is introduced into a surface of an amorphous silicon semiconductor film as a nucleus of crystal growth, which is then subjected to a heat treatment. Owing to the catalysts, crystallization is completed by a heat treatment at about 550.degree. C. for about 4 hours. In consideration of the glass distortion temperature of the glass which is now commonly used as a glass substrate in an active-matrix type liquid crystal display device (for example, about 593.degree. C. for Corning 7059 glass), the above method allowing a heating process to be conducted at such a low temperature is effective. In particular, since a glass substrate is inexpensive, the above method enabling the use of a glass substrate is greatly effective in view of enlargement of substrate area.
The mechanism for the above-described low-temperature crystallization is as follows. First, crystal nuclei with metal elements as their nuclei are generated in the early stage of the heating process. After that, the metal elements act as catalysts to accelerate the crystallization, and the crystal growth proceeds rapidly. In this sense, such metal elements are hereinafter referred to as the catalyst elements.
While the crystalline silicon semiconductor films obtained by crystallizing amorphous silicon semiconductor films using ordinary solid-phase growth methods have a twin crystal structure, the crystalline silicon semiconductor film obtained by accelerating the crystallization using catalyst elements as described above is formed from numerous column-like crystals. Moreover, an internal structure of each of the column-like crystals is in an ideal single crystalline state.
Furthermore, according to a method disclosed in the aforementioned Japanese Laid-Open Patent Publication No. 6-244104, the catalyst elements are selectively introduced into part of an amorphous silicon semiconductor film which is subsequently subjected to a heat treatment. As a result, crystalline silicon semiconductor films and amorphous silicon semiconductor films can be formed selectively in respectively designated regions on the same substrate. When the heat treatment is further continued, a phenomenon occurs where the crystallized regions laterally grow toward the surrounding amorphous regions from the regions in which the catalyst elements are selectively introduced, i.e., in directions parallel to the substrate surface. These crystallized regions grown in the lateral directions are hereinafter referred to as the lateral crystal growth regions.
While crystal nuclei are generated in a random manner in the region into which the catalyst elements are directly introduced, there exist column-like crystals extending substantially along one growth direction in the lateral crystal growth regions. Thus, the lateral crystal growth region has a remarkably excellent crystallinity as compared with the region into which the catalyst elements are introduced. By forming an active region of a semiconductor device utilizing the lateral crystal growth region, a semiconductor device with high performance, in particular with a high operation speed, can be realized.
The disadvantages associated with the conventional crystallization processes disclosed in the aforementioned Japanese Laid-Open Patent Publications will be discussed.
Although the introduced catalyst elements largely contribute to crystallization of an amorphous silicon semiconductor film, the catalyst elements are present at grain boundaries after the completion of crystallization. As a result, the catalyst elements remain in the obtained crystalline silicon semiconductor film. If a large amount of catalyst elements are present in the crystal-line silicon semiconductor films constituting active regions (device formation regions) of the semiconductor devices, the catalyst elements inhibit the realization of high reliability and stable electric characteristics of the semiconductor device. Thus, the presence of a large amount of catalyst elements is not preferable.
In particular, elements effectively acting as catalysts for accelerating the crystallization of an amorphous silicon semiconductor film, such as nickel, form an impurity level in the vicinity of the center of a band gap of the silicon. Therefore, undesirable phenomena such as increase in leak current in an OFF state, a shift of a threshold voltage and deterioration of operational characteristics with elapse of time may occur in view of the operational characteristics of a semiconductor device to be formed.
Therefore, although catalyst elements, such as nickel, for accelerating the crystallization of amorphous silicon are required in a crystallization process, it is desirable to make the amount of catalyst elements contained in the crystalline silicon semiconductor film to be obtained as small as possible. In order to attain this purpose, the amorphous silicon semiconductor film is conventionally crystallized while restricting the amount of catalyst elements to a minimum amount within the necessary range. However, since the amount of catalyst elements required for crystallization is significantly minute, i.e., about 10.sup.13 atoms/cm.sup.2, it is remarkably difficult to restrict the amount of catalyst elements to be introduced to such an extremely low level.
In the case where the catalyst elements are introduced while controlling the amount of catalyst elements to a minute amount, it is difficult to make uniform the amount of catalyst elements to be introduced in the same substrate and/or to stably realize the introduction of a minute amount of catalyst elements between different substrates with good reproducibility. In the case where the amount of catalyst elements introduced into the same substrate or between a plurality of substrates is greatly non-uniform, defective regions are locally generated. In such defective regions, crystal growth does not occur due to a deficiency of catalyst elements, or characteristics of the obtained semiconductor device are adversely affected in a conspicuous manner due to excessive catalyst elements. Consequently, it is difficult to uniformly form a large number of semiconductor devices on the same substrate with the aforementioned conventional method, as an active matrix substrate of a liquid crystal display apparatus on which several hundreds of thousands of TFTs are formed.
In addition, even in the case where the catalyst elements are controlled to be introduced at a minute amount with high reproducibility, the catalyst elements are inevitably present in the device formation region (active region) at a concentration capable of causing crystallization or at a higher concentration. Therefore, it is impossible to completely eliminate the aforementioned adverse effects of the catalyst elements to the operational characteristics of the semiconductor device.